The LHC is back and it’s ready to probe the limits of matter



Since shutting down in early 2013, the most powerful particle accelerator on the planet, the Large Hadron Collider (LHC), has been sitting dormant. Over the past two years this scientific colossus situated at CERN near Geneva, Switzerland, has undergone a series of repairs and upgrades. But now it is ready to reawaken from it’s slumber.

This new era will see a collider with almost double the previous energy, with collisions at 13 TeV. Scaled up into our macroscopic world, the force of these collisions between protons is roughly equivalent to an apple hitting the moon hard enough to create a crater more than 9.5km (6 miles) across.

This new energy frontier will allow researchers to probe beyond the current boundaries of our understanding of the fundamental structure of matter in search of new discoveries.

Detector upgrades

In order to make the most of the new accelerator conditions, the discovery experiments, ATLAS and CMS, have undergone further upgrades during the shutdown period.

Most notably the ATLAS experiment has added an entirely new detector, the Insertable b-Layer, or IBL. This sits very close to the point where the protons slam into each other, creating a cascade of other subatomic particles.


Because the IBL sits closer to the action than the original detectors – which are also still in use – it provides an additional measurement point for particles originating from the collisions, allowing greater accuracy on the resulting measurements.

The IBL will be especially important for identifying heavy particles, such as bottom quarks, which are produced during decays of short-lived particles such as the Higgs boson and are crucial for measurements of the top quark (which decays to a bottom quark and W boson).

Beyond the Higgs boson

During the first run of the LHC in 2012, the ATLAS and CMS experiments ended the 50 year hunt for the Higgs boson, which was predicted by the Standard Model –- a theory governing all particles, forces and interactions.

Having measured the mass of the Higgs boson by looking at the way it decays into other particles, LHC scientists then went one step further. In 2013 they measured the properties of the boson, all of which proved consistent with the predictions of the Standard Model.

Now physicists want to know if the Higgs they found is hiding any surprises. And, perhaps more importantly, what may be lurking beyond it. The increase in LHC energy is coupled with an increase in luminosity, which allows physicists to probe rare events with greater frequency.

This high luminosity in concert with the increase in energy provides an unprecedented environment to interrogate fundamental physics beyond the limits of our current knowledge. The first thing to do with the new data is to study the Higgs boson in depth to see if anything disagrees with prediction.

This could be a window into new physics. Because the Higgs boson loves mass, scientists suspect that it might interact with a range of hidden, massive particles that we cannot see, such as potential candidates for dark matter.

If the Higgs boson is partying with as yet undiscovered particles, physicists hope that their newly improved particle collider and upgraded detector instruments will allow them to crash the party -– and find out something about the attendees.


Supersymmetry, dark matter and other exotica

Even if the Higgs boson were to continue to agree with the Standard Model predictions, the value of its mass is still suggestive of other interesting goings-on in the universe.

When LHC physicists measured the Higgs mass, they found it was lower than what they anticipated. This might make sense if it was being caused – or protected – by one or more particles that exist at a higher mass and were governed by some new “symmetry”.

is one such extension of the Standard Model that would yield additional partners of the known objects that may appear in high-energy LHC collisions.

These particles could act as “bodyguards” of the Higgs, influencing its measured mass. These supersymmetric particles could potentially be produced in the next run of the LHC, perhaps even as early as this year.

One natural consequence of certain supersymmetric models is the production of invisible stable massive particles that are weakly interacting. Such a particle would be an excellent candidate for dark matter, the mysterious invisible matter that we have thus far only detected via its gravitational effect.

Providing clues as to the nature of dark matter is one of the main motivators of the increased energy and intensity of the LHC collisions. Any evidence of dark matter and/or results consistent with supersymmetry would be hugely significant and would open up a new chapter in our understanding of the universe at a fundamental level.

But the experiments must be prepared for any possible signature to be manifested in their collisions, and subsequently mine the data for evidence of exotic resonant structures, extra dimensions or long-lived particles among many other possibilities.

So 2015 promises to be a once in a lifetime opportunity for a generation of physicists who will turn on and commission a machine at unprecedented energies. With new discoveries potentially just around the corner this may well be a defining time in the field of high energy particle physics.

Author Bio: Paul Jackson is a Particle physicist at University of Adelaide